Contamination of Milwaukee's water supply last spring -- the worst outbreak of waterborne disease in U.S. history -- shook public confidence in the nation's drinking water. The unprecedented crisis caused six deaths and nearly 400,000 cases of gastrointestinal illness, sparking demands for measures to prevent similar epidemics. Environmentalists were quick to point out that bacteria are by no means the only threats to drinking-water safety. They noted that many local water systems contain toxic inorganic substances, such as lead, radon and nitrates. The lesson of Milwaukee, environmentalists say, is that federal rules regulating drinking-water safety need strengthening. But water industry officials, citing cost considerations, say the rules should be made more flexible.